This is a long-standing tradition that goes back to Stalin’s times. Any invasion of any country is justified if Russia perceives that country as a threat. No evidence that the threat is real is ever needed.

At this point, the probability is extremely high that Russia will use the ousting of the corrupt President Yanukovich by the citizens of Ukraine as an excuse to claim feeling threatened and invade. When Putin invaded Georgia in the same way in 2008, nobody in the West made as much as a peep (except John McCain who was running for president and made a very strong and important statement). This is telling Putin that nobody will do anything to defend Ukraine either.

Will the world stand mutely by as Putin invades Ukraine and annexes parts of the country?

I do not think that any of NATO countries will raise serious war against the Russian aggression. However, they may ban the entry permits for the Russian officials and expropriate their properties and freeze their accounts in the Western banks. The most of the Russian officials have collected huge treasures, and they keep them in Western countries; so, this may be very serious reason to avoid the war.
I consider the soviet veterans in Kremlin as terrorists, criminals: killers, robbers, frauders.
They do not care about lives of their compatriots, but they may upset, if some part of their treasure become unavailable.
Declarations from the Russian officials look like a paranoia; and we may expect non-rational steps from them. I hope, the new Ukranian leaders realise this.
After to see news about the heroes of Maidan, I expect, that the Ukraine people will defend their county strongly, as Suomi did at the beginning of the World War 2.
If the invasion begins, I expect the occupants to meet the total war, with missiles, airplanes, tanks, warships, Molotov cocktails, barricades, firing tires, snipers, partizans and guerilla.
Neither You nor I plan to go to Ukraine to fight for Ukraine. The all we can do is to alert the Russian soldiers, that in the case of invasion, they will die, with shame death of occupants, in the way, similar as the soldiers of the Red Army used to die at the “Mannerheim line” in 1939.
Anatoly Streliany (Анатолий Стреляный) had formulated the weakness of the Putin’s soldiers. I try to translate it from Russian: “I do not know, how many people – and when – will be ready to die fighting AGAINST Putin and his men. But I know, that FOR them, already today, none is ready to die. No one human.”
This makes the Putin’s army weak, and the war in Ukraine should destroy the RF in a way, similar as the war in Afghanistan in 1980s had destroyed the USSR.

Resume. Russia may loss the war against Ukraine. I doubt if we can give any useful advice to the Ukranian patriots. But there is one thing we can do (and should do) for the Russian soldiers. We should let them know, that their mission to Ukraine will be far from the funny glory of liberators; it will be just shameful deadly aggression. Even if a Russian soldier survives in the Russian-Ukranian war, he will have to bear the reputation of aggressor, occupant, terrorist, killer for all the rest of his life.

Dear Clarissa. Please ask Your government not to give any entry permit to aggressors, or to arrest them in the airport just upon the arrival. Amen!

“Anatoly Streliany (Анатолий Стреляный) had formulated the weakness of the Putin’s soldiers. I try to translate it from Russian: “I do not know, how many people – and when – will be ready to die fighting AGAINST Putin and his men. But I know, that FOR them, already today, none is ready to die. No one human.””

– I don’t know. Ширнармассы still seem to adore Putin. And his propaganda machine is revving them up to a frenzy of anti-Ukrainian sentiment.

I wouldn’t be at all surprised. On another topic, here’s a photo series of Yanukovitch’s “dacha” being opened to the public. His summer house looks like a small resort hotel. The second to last photo is priceless, and the last one isn’t half bad either. http://www.bbc.co.uk/news/world-europe-26307745

I just read a very strange article on Spiegel called “Putins schwerste Entscheidung”, i.e. “Putins hardest/gravest decision” which is written with a lot of understanding and sympathy towards Putin… the article never spells it out, it sounds as if they were preparing the public for the possibility of invasion, and of course especially for the possibility that the EU would not do anything against it…. this is scary.